Vaccuum Die-Casting Method

ABSTRACT

The invention relates to an improved vacuum die-casting, in particular for metals and metal alloys which contain Al, Mg, Zn, and Cu. In order to improve the quality of the components, it is proposed to produce vacuum in the mold cavity and the casting chamber cavity in several phases of the die-casting process. The use of the new method is contemplated for both cold-chamber and hot-chamber die-casting.

The invention relates to a vacuum die-casting method according to the preambles of the first and eighteenth claims.

Die-casting under vacuum has already been used for some times for producing workpieces from metal and metal alloys in particular alloys of metals Al, Mg, Zn, and Cu. By die-casting under vacuum, a high quality of material of work-pieces is achieved as less air and gases is included in the material. For workpieces from, e.g., aluminum, which later are subjected to heat treatment or welding, one can hardly do without vacuum.

In addition, die-casting under vacuum not only possible with use of liquid alloys, but can also be used with other different particular processes. As examples, one can name here: processes in which semi-liquid or doughy materials are used as a cast mass (usually called thixo casting or rheocasting), processes in which the cast mass consists of a material combination (composition) of liquid or semi-liquid metals and non-metal inclusions (MMC), and processes in which liquid material infiltrates a head piece.

Publication EP-OS O 051 310 discloses a process which is known in the industry under the name Vacural© (a registered trademark of Machinenfabric Müiller-Weingarten AG). This and similar later processes are carried out with a tightly closed casting chamber connected with a heat maintaining furnace by a suction pipe. Metal is aspirated into the casting chamber and is metered out by vacuum which is produced in the mold and is precisely controlled.

The vacuum system for such a process consists essentially of a buffer vessel the vacuum in which is produced by a vacuum pump. Sometimes, the vacuum pump is directly connected, sometimes a central vacuum system is used. Further, such vacuum systems also contain interposed valves and filters and pressure measuring tools.

Publication DE-0S 196 45 104 describes a technology which permits to obtain a higher vacuum in the mold cavity when the cavity is connected in succession with two different vacuum buffer vessels, without the need to connect the vessels with each other. This method permits to increase the stability of the process and monitory the process by end pressures of the vessels.

In the simplest form of the vacuum die-casting, the vacuum is generated with a deaeration valve secured on the mold. Only after the casting piston passes past the filing opening during the first casting phase and, thus, breaks the connection to atmosphere, the mold can be put under vacuum. The process time, which remains after the passing of the piston past the deaeration opening, is generally not sufficient to establish pressure balance between the mold cavity and the buffer vessel or to efficiently evacuate the mold cavity with vacuum pumps. Further, the achieved vacuum is adversely affected by the cross-section of the deaeration valve itself and of the connection channels which extend in the mold from the mold cavity to the deaeration valve. Also, constriction in the mold itself can negatively influence the vacuum still further. Leaks, in particular between the casting piston and the casting chamber, result in worse and fluctuating vacuum values. The leaks vary greatly with wear of the piston and the casting chamber and depend from the temperature.

In the process according to EP-0S 051 310, these drawbacks are partially eliminated by applying vacuum already during filling of metal and, thus, more time is available for achieving a lower vacuum. Nevertheless, the metering precision is greatly influenced by the vacuum value, temperature and viscosity of the melt, and condition of the openings through which metal flows. Also, an expensive control is not able to completely overcome difficulties associated with metering.

In addition, this process requires an expensive and careful sealing of the mold and piston because they remain longer under vacuum then at a conventional vacuum die-casting, and leaks have a greater influence.

Accordingly, an object of the invention is to provide an improved die-casting method with which larger deaeration cross-sections are possible so that a higher vacuum is achieved. The requirements to the piston and the mold with regard to vacuum-tightness should be reduced. The metering precision during filling of the casting chamber with metal or metal alloy should be increased.

This object is achieved with a method of die-casting of metal alloys which form a cast mass under application of vacuum. It comprises the steps of:

-   -   a filling the casting chamber (6) with the cast mass (8),     -   b sealing a casting chamber cavity (5) against atmosphere,         whereby the steps a and b can be exchanged, and is characterized         in that after sealing and separating the casting chamber cavity         (5) from atmosphere, a first vacuum phase is carried out, and a         second or further vacuum phases are carried out after separation         of the casting chamber cavity (5) from a first conduit to a         vacuum system (12).

Further claims disclose further embodiments.

The object is also achieved with a hot-chamber die-casting method wherein a vacuum system is provided with at least two vacuum conduits, and which includes the steps of:

-   -   1. Displacing a casting piston so that a filling opening (72) is         closed; 2. Producing vacuum in a casting ladle (74), an         intermediate piece (75), a mouthpiece (76) and a mold cavity via         a first vacuum conduit (12), characterized in that 3) a first         vacuum phase is carried out after the casting piston passes past         the filling opening (12); 4) thereafter, the connection with the         first vacuum conduit (12) is broken; and 5) finally, a second         vacuum phase is carried out.

The new method according to claim 1 or claim 18 has at least two vacuum phases in the total casting cycle, in which vacuum is produced. In the first phase, the atmosphere through the casting chamber is exhausted. This permits to use connections which have much larger cross-sections. Contrary to exhaust through the mold or the mold valve, noticeably better conductance in the vacuum system and, thereby, a better end pressure are achieved. At that, the better conductance permits to reduce the time of the first phase. Because the greater conductance permits to achieve a better vacuum sooner, it is possible, in comparison with the method according to EP-OS 0 051 310, to tolerate, at a much greater degree, the leaks in the mold and along the piston. The casting chamber, which is closable according to the invention, enables to carry out the metal metering with high precision independent from vacuum.

According to the invention, in a second phase, the vacuum is generated through the mold, whereby the achieved end pressure is improved. This phase takes place when the cavity with the metal melt is separated from the conduit to the vacuum system of the first phase. This can take place as a result of displacement of the casting piston or closing of a valve in the conduit.

With a low end pressure in the mold, the amount of gases confined in the structure of the produced component is reduced. High metal pressures, which were required up to now, can, therefore, be reduced because the remaining residual gas must be compressed to a lesser degree to achieve the same quality of the structure. The method enables to improve the quality of the component at die-casting of metal and metal alloys.

In further embodiments, this method can be improved. By using a vacuum system with a buffer vessel for a vacuum phase, it is possible to achieve such low pressures in the mold that closing of the deaeration valve is possible before the metal reaches the valve.

According to a further embodiment, an improved end pressure is achieved when the space behind the casting piston is likewise evacuated during the first and/or the second phase. In one of the embodiments, this can be achieved when the space behind the piston forms a whole with a hood which closes the chamber or is connected therewith. A casting chamber, which is closed with a hood has a further advantage consisting in that after the hood is closed, vacuum generation can be started immediately with the first phase, while the piston has not yet closed the filling opening. Thereby, the process time can be reduced, while the piston can have a high speed during the first casting phase. In addition, more time remains for carrying out the second phase.

In yet another embodiment, no use is made of a closable hood. The separation of the casting chamber cavity from atmosphere takes place, while the piston closes the filling opening. The casting chamber is connected with the conduit of the first vacuum phase by a further big-dimensioned opening. The first vacuum phase is produced before the piston closes the second opening. The piston can have one or several sealing rings. This embodiment can be incorporated in existing plants without any substantial expenses.

Alternatively, in one of the embodiments, the sealing and separation of the casting chamber from the atmosphere take place while the two mold halves are assembled together, i.e., the mold is closed. Thereby, the number of the necessary components and openings in the casting chamber is reduced.

The sealing and separation of the casting chamber cavity from the atmosphere can in other embodiments take place, while the casting chamber is connected with the mold. Thereby, it is possible then to separate the casting chamber and the mold from each other and to fill the casting chamber in this condition of the assembly. After filling, the mold is connected with the casting chamber. This can e.g., take place at sides of the mold with a movable ram, slide, or piston at the chamber end or in cast-in channel. Alternatively, e.g., a vertical or pivotal casting chamber can be formed, as is conventional in many “squeeze casting” plants.

Further, the sealing and separation of the casting chamber cavity from the atmosphere takes place with a movable cover that closes the filling opening. This embodiment can be rapidly closed and is particularly advantageous when metal is metered through the filling opening into the casting chamber through the pouring spout. The pouring spout or a connection tube between the pouring spout and the casting chamber is likewise formed movable. The combination of a metering furnace with the pouring spout is actually a most recommended metering system for most of foundries.

In one of the embodiments in which the metering pot is mounted on the filling opening, the casting chamber cavity and the atmosphere are separated from each other by a (heated) closable metering pot. Here, the metering pot serves as a storage vessel for a to-be-metered cast mass. Thereby, the metering process is made independent from the remainder of the casting process. The casting chamber can, thus, be filed with metal before, during, or after vacuumization or feeding of protective or reaction gases. The metering pot itself is filled through a closable opening (cover, tube, metering piston . . . ) and can itself be vacuumized or subjected to action of protective or reaction gases.

Independent of the used method, in order to separate the casting chamber cavity from the atmosphere, the first conduit to the vacuum system can be connected, otherwise as descried above, to a casting chamber cover or directly to the casting chamber, also to the cast-in channel or a connected therewith, big-dimensioned channel in the mold. An additional valve must be mounted at the admission point in the mold in order to prevent penetration of the metal in the conduit.

According to a further embodiment, the inventive method can be used in a hot-chamber die-casting process. With this process, mostly, magnesium, zinc, or lead-based metal alloys are cast. Hear, a casting ladle (gooseneck) is immersed in metal in a heat maintaining furnace. It includes, in addition to a casting chamber, a gooseneck-shaped channel which is connected with a mold via an intermediate piece and a mouthpiece. This unit (gooseneck, intermediate piece, mouthpiece) in a broad sense can be seen as analogue of the casting chamber cavity in the cold-chamber process. By a connection to this unit, a first vacuum phase can be introduced, so that the already described invention is usable in hot-chamber die-casting process. With large cross-sections, it is possible to produce vacuum so rapidly that it can take place shortly before the charging process. Thereby, it is prevented that metal is aspirated into the mold cavity by the produced vacuum already before the charging process itself.

The method according to the present invention can advantageously be used for metal alloys which primarily contain aluminum because it is with this metal, the presence of large shares of air in the structure makes further processing (e.g., welding) more difficult if not impossible.

The invention will be explained in detail based on the following description and drawings.

FIG. 1 a cross-sectional view of a die-casting machine suitable for carrying out the method;

FIG. 2 a cross-sectional view of the casting chamber. The sections a through c show different phases of the process;

FIG. 3 an embodiment of a vacuum system with several buffer vessels;

FIG. 4 an embodiment with a direct connection to the casting chamber (separation from atmosphere with the casting piston);

FIG. 5 an embodiment with a metering spout and a closing mechanism on the filling opening;

FIG. 6 an embodiment with a closable metering pot; and

FIG. 7 an embodiment of a hot-chamber die-casting.

FIG. 1 shows a basic embodiment of a die-casting machine suitable for carrying out the inventive method. A mold 21 is clamped between two plates 22. A to-be-finished workpiece is obtained by solidification of metal or metal alloy in the mold cavity 10. A piston 3, which presses a mass of liquid metal 8 into the mold cavity as a result of its linear movement, is displaceable in a casting chamber 6. The casting chamber is provided with a filing opening 4 through which the liquid metal is fed before the first vacuum phase. A hood 7 vacuum-tightly seals the casting chamber from outside. The cavity within the hood is connected by a port 11 and a first conduit 12, in which the a valve 13 is located, with a vacuum system 20. The vacuum system can be formed by an arrangement of vacuum pumps and/or buffer vessels. Vacuum is introduced over the first conduit during the first vacuum phase. A second conduit 15 is connected with a valve 16 that connects a deaeration valve 14, which is mounted on the mold with the vacuum system 20. Over the second conduit, the vacuum is introduced during the second vacuum phase.

The method with two vacuum phases will be explained in detail with reference to FIG. 2. FIG. 2 a shows the first step during which metal in form of a cast mass 8 is fed into the casting chamber 6. In this step, the hood 7 is pulled back, releasing the filling opening 4. In the next step in FIG. 2 b, the hood 7 is pushed forward and vacuum-tightly closes the casting chamber cavity 5 at the piston side, the sealing is effected with a circumferential seal 2 provided on the piston rod 1. Only then, the vacuum system generates, over the port 11, vacuum which corresponds to the first vacuum phase. This port can have a large conductance because practically there are no spacial limitations. Thereby, the mold cavity and the casting chamber are effectively and rapidly evacuated. Between the second and third steps, the piston 3 is pushed forward so that at the end, the filling opening 4 is closed. Thereby, the volume above the cast mass is separated from the cavity beneath the hood 7 and, therefore, from the vacuum conduit means 11. In the third step, the mold is evacuated via the valve 14 and the second conduit 15 in accordance with the second vacuum phase. This phase ends when the valve 14 is closed.

The phases can be explained with reference to the positions of valves 13, 14 and 16. During filling of the casting chamber with the cast mass, the valves are closed. As soon as the hood 7 is pushed forward, the casting chamber becomes sealed against the atmosphere, and the valve 13 opens, the first vacuum phase takes place. After the displacement of the piston, the filling opening becomes closed, and the vales 14 and 15 open (in most cases, the valve 14 is already open), and the second vacuum phase takes place. When the mold is filled with metal, the valves 14 and 16 are closed. There, most of the aeration valves are closed by the metal itself.

In a simplified embodiment, the separation of the first conduit is carried out only by closing the valve 13, which permits to simplify the construction. After closing, the second vacuum phase can be carried out, independent on the position of the piston.

A further embodiment is associated with filing of the casting chamber with metal. This need not necessarily take place before the first vacuum phase. It is conceivable to use a vacuum phase for metering of the cast mass, with the metal being aspirated into the casting chamber by vacuum, e.g., through a riser.

The metering of the cast mass should not take place in the first vacuum phase but takes place in subsequent vacuum phases. Thereby, the reaction of the cast mass with gases in the casting chamber can be reduced, as those are reduced during the vacuum phase before filling.

In an advantageous embodiment, instead of one vacuum phase, protective or reaction gases are fed into the casting chamber and/or the mold cavity. As a result, reactions of the cast mass with gases are prevented, e.g., oxidation of the cast mass surface is prevented to a great extent. Also, gases can be fed which purposefully react with the cast mass and improve the properties of the material.

In an advantageous embodiment, after the first vacuum phase, no aeration of the cavity within the hood takes place. The vacuum during one or several further vacuum phases can remain. Therefore, the leaks along the piston are less critical, and possible leaks do not lead to a dramatic deterioration of vacuum.

The valve 13 can remain open. With this system, particularly low end pressure in the mold can be achieved, so that it is possible to close the deaeration valve 14 before metal reaches it.

In a further embodiment, the sealing of the casting chamber against atmosphere is effected not by linear displacement of the hood as in FIG. 2. Rather, in this embodiment, the hood is formed as a part rotatable about the axis of the casting chamber 6 and the piston rod 1. The filling opening becomes closes as a result of this rotation. Such a hood proved to be advantageous, having a shorter length and a shorter closing time.

A suitable embodiment of a vacuum system 20 is shown in FIG. 3. The first conduit 12, which is used in the first vacuum phase, and the second conduit 15, which is used in the second vacuum phase, are connected, respectively, with vacuum buffer vessels 17 and 18. The vessels are evacuated by vacuum pumps. Advantageously, there is provided a common arrangement of vacuum pumps or a common pump 19 with which the vessels can be evacuated independently from each other in accordance with switching of the valve 23.

Advantageously, a separate vacuum buffer vessel is provided for each phase. FIG. 4 shows an embodiment in which the casting chamber cavity 5 is sealed against atmosphere by the piston 3. FIG. 4 a shows a process step after the liquid metal 8 has filled the casting chamber 6. After the piston 3 passed past the filling opening 4, the casting chamber cavity 5 becomes sealed against atmosphere, and the first vacuum phase can be introduced through the port 42 of the first conduit 12 into the casting chamber 6. This step is shown in FIG. 4 b. When the valve 13 in the first conduit becomes closed or the piston passes past the port 42, the second vacuum phase can be introduced through the deaeration valve 14 and the second conduit 15. This condition is shown in FIG. 4 c. While retaining vacuum, the metal fills the mold cavity 10 under the action of the piston 3, a shown in FIG. 4 a.

Alternatively to the port 42, a connection can be formed on the cast channel 41 or be realized in a form of a channel connectable herewith. The cast channel 41 has, as a rule, a large cross-section in the direction of the casting chamber cavity 5. This connection, naturally is, closed with additional valve (analogous to the deaeration valve 14), so that no metal can penetrate in the first vacuum conduit 12. After this valve or the valve 13 is closed, the second vacuum phase can be introduced.

FIG. 5 shows an embodiment in which the filling opening 4 of the casting chamber 6 can be closed with a lid 53, and the metal 8 is metered into the casting chamber 6 over a movable pouring spout 51. FIG. 5 shows the process during metering of the metal. After the casting chamber 6 is filled with the metal 8, the pouring spout 51 is lifted off the filling opening 4 vertically upward, so that the lid 53 can pivot beneath the pouring spout 51 and can be lowered onto a lid seat 52 after a short vertical movement. This seals the filling opening 4, and the first vacuum phase can be introduced through the first conduit 12.

The vertical movement of the pouring spout 51 and the rotational movement of the lid prevents the sealing surface between the lid and the lid seat from contact with the hot metal and its contamination. The seal 54 is preferably mounted on the underside of the lid 53. In order to eventually collect the draining metal, a drain metal sheet 55 is mounted above the lid 53.

Alternatively to movement of the pouring spout 51, a short movable feed tube 58 can be provided, which is displaced downwardly during metering to bridge the distance between the outlet of the pouring spout 51 and the filling opening 4. Thereby, sidewise flow of the metal or its splashing is prevented.

The cavity behind the piston is likewise connected, in the embodiment shown in FIG. 5, by a connection 57 with the first vacuum conduit 12.

The embodiment of FIG. 6 represent a solution that contemplates filling of the casting chamber 6 with metal 8 using the metering pot 61. The metering pot simultaneously provides for separation of the casting chamber cavity 5 from atmosphere.

The metering pot is filled with metal in the first step (FIG. 6 a). After the mold 21 is closed, the first vacuum phase can already be introduced at a very early point in time (FIG. 6 b). This takes place in this embodiment over the first vacuum phase conduit 12. Independent therefrom, alternatively, the metering pot cavity 63 can be brought to a certain underpressure. By pulling the plug 62, the metal is metered into the casting chamber 6 through the filling opening 4. An eventually occurring (positive) pressure difference between the metering pot cavity 63 and the casting chamber cavity 5 would accelerate the metering process. After filling with metal and closing the valve (13) in the first conduit, immediately thereafter a further vacuum phase is introduced.

FIG. 6 a shows a modification of the metering pot 61 with a closable lid 64. The lid can be open after the plug 62 reliably closes the through-opening. It makes sense to wait with opening of the lid until the piston 3 passes past the filling opening 4 in order not to jeopardize the tightness of the casting chamber cavity 5 by a possible leakage at the plug 62. Then, the metering pot 61 is filled again with the cast mass 8 by an arbitrary point in time. In case, e.g., the metering pot cavity 63 is under a protective gas, the metering pot 61 can be filled through a closable tube or a metering piston instead through a closable lid 64.

In another embodiment, instead of a metering pot, a closable metering tube or a metering piston can be excused.

An embodiment for a hot-chamber method is shown in FIG. 7. As it is conventional with the use of the hot-chamber method, the casting ladle (gooseneck) 74 is immersed in metal that fills a heat maintaining furnace 71. The casting ladle 74 has a casting chamber 73 having a filing opening 72 that connects the casting chamber 72 with the metal bath in the heat maintaining furnace. The casting ladle has also a gooseneck-shaped channel that connects the casting ladle through an intermediate piece 75 and a mouthpiece (heated) 76 with the mold 21. FIG. 7 a shows an initial position in which the piston 3 is in its initial position and the metal in the casting ladle 74 is at the same level as in the heat maintaining furnace 71. In a second step, the piston 3 closes the filling opening 72 so that no metal can flow. At this time, the conduit 12 can introduce a first vacuum phase (FIG. 1 b). Before the metal raises up to the intermediate piece 75, an additional valve (piston) 77 becomes closed, so that no metal can flow in the first conduit 12 to the vacuum system. By this time, a second vacuum phase can be introduced over the conventional deaeration valve and the second conduit 15. This step is shown in FIG. 7 c. In the last FIG. 7 d, the piston 3 advances the metal further into the mold over the intermediate piece 75 and the mouthpiece 76. 

1. A method of die-casting of metal, metal alloys, which form a cast mass, under application of vacuum, comprising the steps of: a: filling the casting chamber (6) with the cast mass (8), b: sealing a casting chamber cavity (5) against atmosphere, whereby the steps a and b can be exchanged, characterized in that after sealing and separating the casting chamber cavity (5) from atmosphere, a first vacuum phase is carried out, a second or further vacuum phases are carried out after separation of the casting chamber cavity (5) from a first conduit to vacuum system (12).
 2. A die-casting method according to claim 1, wherein sealing and separation of the casting chamber cavity (5) from atmosphere takes place by closing the mold (21).
 3. A method according to claim 1, characterized in that sealing and separation of the casting chamber cavity (5) from atmosphere is carried out by connecting the casting chamber (6) with the mold.
 4. A method according to claim 1 characterized in that the sealing and separation of the casting chamber (5) is carried out by closing a filling opening (4) with a casting piston (3).
 5. A method according to claim 1, characterized in that separation of the casting chamber cavity (5) from atmosphere is carried out with a cover closable from outside.
 6. A method according to claim 5, characterized in that sealing of the casting chamber cavity (5) is carried out by a linear movement of a hood (7).
 7. A method according to claim 5, characterized in that sealing of the casting chamber cavity (5) is carried out by a linear movement of a hood (7).
 8. A method according to claim 1, characterized in that metering of the cast mass (8) takes place before carrying out the first vacuum phase.
 9. A method according to claim 1, characterized in that metering of the cast mass (8) takes place during or by means of a vacuum phase.
 10. A method according to claim 1, characterized in that metering of the cast mass (8) takes place after carrying out a preceding vacuum phase.
 11. A method according to claim 1, characterized in that instead of a vacuum phase, protective or reaction gases are fed into the casting chamber and/or a mold cavity or such a phase is interposed.
 12. A method according to claim 1, characterized in that the filling opening (4) is closed by displacement of the piston.
 13. A method according to claim 1, characterized in that separation of the casting chamber cavity (5) from the first conduit to the vacuum system (12) is carried out by closing a valve (13).
 14. A method according to claim 1, characterized in that separation of the casting chamber cavity (5) from the first conduit to the vacuum system (12) is carried out by displacement of the casting piston (3).
 15. A method according to claim 1, characterized in that vacuum, which remains behind the casting piston (3) after the first vacuum phase, remains in at least one further phase.
 16. A method according to claim 1, characterized in that vacuum, which is necessary for several vacuum phases, is produced with a respective buffer vessel (17, 18).
 17. A method according to claim 16, characterized in that the buffer vessels (17, 18) are evacuated by a common arrangement of vacuum pumps (19).
 18. A hot-chamber die-casting method, wherein a vacuum system is provided with at least two vacuum conduits, including the steps of:
 1. Displacing a casting piston so that a filing opening (72) is closed
 2. Producing vacuum in a casting ladle (74), an intermediate piece (75), a mouthpiece (76) and a mold cavity via a first vacuum conduit (12) characterized in that
 3. a first vacuum phase is carried out after the casting piston passes past the filling opening (3)
 4. thereafter, the connection with the first vacuum conduit (12) is broken
 5. finally, a second vacuum phase is carried out.
 19. A hot-chamber die-casting method according to claim 18, characterized in that an additional valve (77) prevents penetration of metal in the first conduit (12).
 20. A hot-chamber die-casting method according to claim 19, characterized in that the additional valve (77) is provided on the intermediate piece (75) located between the casting ladder (74) and the mouthpiece (76).
 21. A method according to claim 17, characterized in that cast mass (8) contains aluminum as a major component. 